How do I make a new class by adding methods to an already existing library class in Java?
composition, design-patterns, inheritance, java, oop
Solution
If you aren't actually changing the representation of `BigDecimal` itself, and are just adding some helper methods, then you can have another class that contains your helper methods statically. ie
class BigDecimalMethods{
public static BigDecimal reciprocal(BigDecimal bd){
}
//etc
}
Problem
I am trying to add some convenience methods to Java's BigDecimal and create a CustomBigDecimal class. Say I want to add a method reciprocal(). I have tried doing this with inheritence as follows: ``` public class CustomBigDecimal extends BigDecimal { ..... ..... public CustomBigDecimal reciprocal() { ..... } } CustomBigDecimal foo1 = new CustomBigDecimal(1); CustomBigDecimal foo2 = new CustomBigDecimal(2); CustomBigDecimal foo2 = foo1.add(foo2); //cannot cast superclass to subclass ``` The problem with this approach is that I cannot cast a superclass to subclass (for reasons I am well aware of). And all the methods of the superclass return a BigDecimal. I have thought of a solution to solve this using composition as follows: ``` public class CustomBigDecimal { private BigDecimal val; CustomBigDecimal(BigDecimal val) { this.val = val; } ...... ...... public CustomBigDecimal add(CustomBigDecimal augend) { return new CustomBigDecimal(val.add(augend.getBigDecimal())); } ..... ..... public CustomBigDecimal reciprocal() { .... } } ``` But if I go with the second approach, I have to write every method of BigDecimal. Is there a better approach to solve this problem?